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Disconnecting the battery for winter storage

I have always disconnected the negative terminal of the battery when storing my collector cars during the New England winters. Our owner's manual suggests this approach. Is there any down side to doing this?
LarryG

From what I understood is that even an 'intelligent' (trickle or jogger)charger will eventually decrease the lifespan of the battery if constantly connected.
What I do now is; fully charge the battery (at low current: 500-1000 mA), voltage is about 12,70 V, from that point I just let it it discharge. With the alarmsystem on this will take about 8 (!) weeks. During this time I also check the voltage. Normally a battery is empty when the voltage is below 11,6 V but my Z will even start when the voltage reads 10,9 V (!). Deep discharging is of course very bad for the battery, so when voltage is below 10,9 I charge it again.....

When the battery is too low to start, I use a Black and Decker starting help:

Even though it has a 80 Amp setting, only charging at 10-30 A will get the startengine running....!

There's no issue with disconnecting the battery as recommended by the manual. After all, the people that wrote the manual built the car. So, have confidence in the procedures described by BMW's own publications.

If you do this regularly, you might want to install a disconnect for your convenience. I've considered it, but my gel battery is always connected to the Odyssey charger when the car is not in motion.

1. Use a battery tender. Do NOT charge your battery and then let it discharge as you are cycling the battery which is harder on it than using the tender.

2. If you are going to repeatedly disconnect your battery, use a battery disconnect. It is installed in place of the cable, then the cable is attached to it. When you want to disconnect the battery, you just back off the knob without removing anything. Simple.

I have a vague memory that there had been some issues in the past with the key/lock/alarm/electrics when the battery is left disconnected?
Maybe it was on one of my other cars, but I thought it was a Z8 issue. Ring any bells?

Andrew Macpherson

Expert Z8 Inspections, with full support for both Z8 sale and purchases.

It does not ring any with me however, from a general standpoint, many systems / cars rely on the battery in order to retain certain information and operational capabilities. If they are allowed to loose that information via a disconnected battery, the information may need to be restored in order for the vehicle to operate normally.

If you can find the time to check yout battery voltage from time to time and you do NOT let it drop below 10,0 Volt 'normal' lead batteries will last a very long time.
The worst that can happen to a lead battery is of course deep discharge, so 'normally' we try to keep the battery at the 'fully charged' end of the spectrum.
Tenders/joggers/refreshers will try to simulate a 'normal use cycle'. In this process it will discharge the battery (in my refresher this is done with about 70mA). After the voltage drops below a certain point or within 15 minutes, it will charge it again with 100-200 mA. This process is repeated endlessly. Great advantage of these tenders/joggers is that a any given time we are able to start our cars, slight disadvantge is that the continuous charging-discharging process will (slightly) cause the battery to build up leadsulfate on it's plates. Because we have fairly large capacity batteries (90ah) this process luckily will take a rather long time

@Gunguy: are you happy with your gel-battery? Why did you replace your lead battery with a gel type?

I couldn't be happier with the Odyssey line of batteries. I've used them in many vehicles. For performance cars, I particularly like the lighter weight, and the ability to mount them in any orientation has been helpful at the track.